In a catalyst apparatus for purifying exhaust gas discharged from an internal combustion engine of an automobile or the like, various catalysts have been used in response to object thereof. As a major catalyst component thereof, there is a platinum group metal, and usually it is used by being supported, in high dispersion, onto a refractory inorganic oxide having high surface area, such as activated alumina (refer to PATENT DOCUMENT 1).
As for the platinum group metal as the catalyst component, platinum (Pt), palladium (Pd), and rhodium (Rh) have been known, which have been used widely as the catalyst for purifying exhaust gas discharged from an internal combustion engine of an automobile or the like. In the aforementioned TWC, a catalyst activated species superior in oxidation activity, of Pt, Pd or the like, and Rh superior in purification activity for NOx are used in combination, in many cases. In recent years, regulations on hazardous substances contained in exhaust gas, in particular, NOx, have become more and more severe. Accordingly, it is necessary to use effectively Rh superior in purification activity for NOx, as the catalyst component, however, Rh is scarce in production amount and high price, which has caused price hike in recent market. Therefore, it is preferable to decrease used amount of Rh, as a catalyst activated specie, in view of resource protection, as well as cost.
In addition, as for the catalyst for purifying exhaust gas, to attain enhancement of further purification performance, the addition of various promoter components, other than the platinum group metal, to the catalyst has been investigated. As these promoter components, an Oxygen Storage Component (OSC), or an alkaline earth metal or zirconium oxide, zeolite and the like have been known.
Among these, OSC is the one which stores and discharges oxygen in exhaust gas, and cerium oxide has been known. Cerium oxide stores oxygen as CeO2, when oxygen concentration in exhaust gas is high, and discharges oxygen by being converted to Ce2O3, when oxygen concentration is low. Oxygen discharged is activated oxygen, which promotes purification of HC and CO, by being utilized in oxidation action by Pt or Pd. In addition, OSC also acts to mitigate oxygen concentration change in exhaust gas, by storage and discharge of oxygen. By this action, purification performance for exhaust gas is enhanced in TWC. TWC is the one which performs oxidation and reduction by one catalyst, and has a range of exhaust gas components suitable for purification. This range is called a window, and air/fuel ratio is controlled so as to attain theoretical air-fuel ratio, called stoichiometry. By mitigating change in oxygen concentration by oxygen storage•discharge of OSC, when oxygen concentration in exhaust gas is out of this theoretical range, exhaust gas components are maintained in this window region, and purification of exhaust gas can be performed effectively.
As such a cerium oxide, a pure cerium oxide can also be used, however, it is often used as a composite oxide with zirconium (refer to PATENT DOCUMENT 2). A cerium-zirconium composite oxide is said to have high heat resistance and high storage and discharge rate of oxygen. The reason is considered that a crystal structure of the cerium-zirconium composite oxide is stable, and also does not inhibit action of a cerium oxide, which is a main OSC component, thus functions as the OSC up to inside of the particle.
On the other hand, in purification of NOx by Rh, it is considered that, for example, a steam reforming reaction and a CO+NO reaction are promoted via the Rh component as shown below and to purify NOx.HC+H2O-----→COx+H2  (1)H2+NOx-----→N2+H2O  (2)CO+NO-----→CO2+1/2N2  (3)
And, it has become known technology that the zirconium oxide promotes the steam reforming reaction and the CO+NO reaction, when it is used together with the Rh component (refer to PATENT LITERATURE 3).
As the promoter component, other than this, an alkaline earth metal such as a Ba component or the like has also been known (refer to PATENT LITERATURE 4). The Ba component temporarily stores NOx contained in exhaust gas, and purifies the stored NOx by reducing to N2 by a reducing component contained in exhaust gas.
In general, NOx is generated in a large quantity, when fuel supplied to an engine is less, amount of air is more, or combustion temperature is high. The Ba component temporarily absorbs NOx generated in this way.
And, NOx absorbed onto the Ba component is discharged from the Ba component, when NOx concentration becomes low, and CO concentration becomes high in exhaust gas. This is caused by a reaction of Ba (NO3)2 with CO to be converted to BaCO3, and can be said chemical equilibrium. NOx discharged from the Ba component, as described above, is purified by reduction, by a reaction with a reducing component at the Rh component surface.
Such a promoter component may be used in combination of two or more and, for example, there has been known TWC, where the Ba component (barium acetate or barium nitrate) and cerium oxide are used (refer to PATENT LITERATURE 5), or TWC where barium sulfate and a cerium-zirconium composite oxide are used, proposed by the present inventor, are used (refer to PATENT LITERATURE 6).
However, it has been reported that purification performance is decreased when a combination of catalyst materials is wrong and, for example, presence of the Rh component and the Ba component in the same composition decreases purification performance for NOx (refer to PATENT LITERATURE 7). The reason for this is considered that, because the alkaline earth metal component has action of storing NOx, purification action for NOx in the Rh component is interfered, or an oxidized Rh structure is stabilized by electron donating action from Ba to Rh.
Therefore, it has been proposed to enhance purification performance for NOx and heat resistance, by supporting the Rh component and the Ba component onto alumina in a separated state (refer to PATENT LITERATURE 8). In this Literature, there is no description on what degree the Rh component and the Ba component are separated in the catalyst layer, however, in the case of using water-soluble Ba acetate as a Ba source, the Ba component dissolves into slurry, and thus it cannot be said that it is sufficiently separated from the Rh component. As a result, the Rh component and the Ba component come close and thus purification performance for NOx results in to be decreased.
In addition, there have been challenged a method for adding a rare earth oxide, such as neodymium oxide, praseodymium oxide, to alumina, in order to enhance heat resistance of alumina, which is a base material, and CeO2, which is an OSC component (refer to PATENT LITERATURE 9), or adding a rare earth oxide such as lanthanum oxide, neodymium oxide, to cerium oxide (refer to PATENT LITERATURE 10), as well as supporting Rh, Pd or the like, which is a noble metal, onto both of alumina particles having heat resistance enhanced by lanthanum oxide, zirconia or the like, and CeO2 fine particles having heat resistance enhanced by neodymium oxide, zirconia or the like (refer to PATENT LITERATURE 11).
Other than this, there has been challenged to coat zirconia with an oxide of a metal selected from a group consisting of iron, gallium, scandium, aluminum, indium, lutetium, ytterbium, thulium, yttrium and holmium, in the case of using zirconia, as a base material, to suppress a solid solution of Rh, so as to suppress easy transfer of Rh to the zirconia surface and particle growth (refer to PATENT LITERATURE 12). In this way, particle growth or transfer can be suppressed by forming a relatively thin metal oxide layer on zirconia, and generating interaction between Rh and these meta oxides, however, in the case of forming an alumina layer, particle growth and transfer suppression effect of Rh has not been obtained, as compared with the case of forming an oxide of iron, gallium, scandium or the like.
In this way, there are various combinations of the catalyst components, and complicated reaction routes are taken by mutual interaction of the catalyst components, and thus by overall investigation on these, a combination of the catalyst components which exerts purification action most has been searched.
In addition, in order to exert catalytic performance effectively, countermeasure has been taken such as the catalyst is arranged separately in exhaust gas, at the upstream side and the downstream side, or a plurality of the catalyst layers are installed at a substrate surface.
This aims at well utilization of characteristics of the catalyst for purifying exhaust gas, in response to strengthening of exhaust gas regulations. For example, each optimum position is required to be set for the noble metal of platinum, palladium or rhodium, in response to durability (heat resistance, atmosphere resistance, poisoning resistance), catalyst characteristics (oxidation activity, reduction activity) or the like, therefore Rh or Pt/Rh is arranged at the surface layer, and Pd or Pd/Rh is arranged at the lower layer.
However, exposure to high temperature exhaust gas from an engine may sometimes results in transfer of Rh arranged at the surface layer to the lower layer, or decrease in catalytic performance, caused by particle growth.
Still more, regulations on exhaust gas have become severer and severer in recent years, and advent of such a catalyst has been desired that enables to exert more superior purification performance for exhaust gas, by using a plurality of catalysts, or by using a catalyst having a plurality of catalyst layers. Among exhaust gas, regulation value for, in particular, NO in exhaust gas has become severer, however, the conventional TWC could incur not only decrease in purification performance for NOx, by transfer of Rh from the surface layer having high gas contact to the lower layer, but impair of Pd performance, by making an alloy with Pd at the lower layer, as described above.